The invention relates to a method to the preamble of attached for implementing a packet-form telecommunication connection.
In principle, the method of the invention can be applied in any packet switched telecommunication network in which an individual data packet has a certain minimum size, but the invention, however, is primarily intended for an ATM network (Asynchronous Transfer Mode), in which a data packet has a certain standard size.
As previously known, ATM is a new connection-oriented packet switching technique, in which the problems of conventional packet networks have been solved by proceeding to use short packets of standard length (53 bytes), known as cells. Each cell consists of a 48-byte payload part and a 5-byte heading. The ATM technique, however, will not be described in greater detail herein, since the method of the invention does not require any solutions specific for the ATM technique.
Packaging of a packet--e.g. an ATM cell--for sending in a packet network causes a certain delay; the delay naturally being the greater, the slower is the bit stream from which the packets are formed.
The packaging delay may thus become even too long for time; critical applications with a slow bit rate. For example, an acceptable delay for speech encoded by a speech codec to a rate of 8 kbit/s is about 25 ms at both ends of the connection (M. J., McTiffin et al., IEEE Journal on Selected Areas in Communications, 12,900, (1994)). Filling of a single ATM cell at this rate, however, causes a delay of about 48 ms. The delay is problematic especially in situations where the persons talking take turns. For example, if subscriber A asks a question from subscriber B, it first takes 48 ms until an ATM cell is filled, and then another 48 ms at the opposite end before the response given by subscriber B is sent back. The two-way packaging delay alone is thus about 0.1 s, which is very irritating to subscriber A (since he or she wonders why subscriber B is not responding to the question asked).
The above described delay problem is solved in ATM networks by filling the cells only in part. For example, if the payload part of the cells is filled only in half with an actual payload signal, the packaging delay in the above provided example is 24 ms, which is acceptable. The drawback of such a solution, however, is that some of the transmission capacity of the network is wasted.
Another previously used way of reducing the packaging delay is to multiplex several slow bit streams into one and the same ATM cell. When a solution like this is used, these different `mini cells` have to be demultiplexed from the ATM cell in the ATM network, whereby many of the advantages offered by the standard ATM technique based on fixed cell size are lost. Another drawback of the multiplexing solution is that it requires permanent allocation of channels to traffic sources, independently of whether or not data is transmitted.
A packaging delay may sometimes also delay the delivery of e.g., control or supervision information too long. Above all, this may happen in situations where alarm type information has to be transmitted via the packet network to a receiving end. Problems caused by packaging delay may thus also relate to control information transmitted in the network as well as to subscriber data.